Thin Layer Densitometric Determination of Gallic Acid and Gallotannins in Wine and Cider

Abstract

Free gallic acid and total gallotannins are determined in wine and cider by thin layer densitometry. The beverage is extracted with ethyl acetate and the extract is subjected to thin layer chromatography on silica gel. After elution with chloroform-ethyl formate-formic acid (50+40+ 10), the dry plate is sprayed uniformly with a freshly prepared solution of 0.3% aqueous FeCl₃ and 0.3% aqueous K₃FeCN₆. If the gallic acid spot appears (R_f 0.28), densitometry readings are taken at 600 nm, and free gallic acid (GA_F) is calculated from the formula GA_F = GA (μg, from calibration curve) × 1.25. The calibration curve was obtained by plotting thin layer densitometric readings at 600 nm (prussian blue) vs. μg gallic acid. In a second method, the beverage is hydrolyzed, and extraction and densitometry are performed as before to give “total gallic acid after hydrolysis:” GA_H (mg/L) = GA (μg, from calibration curve) × 25. Total gallotannins (GALL_T) are calculated from the formula: GALL_T (mg/L) = GAH – GA_F. Results for 10 wines and 5 ciders are presented and briefly discussed.